1. Field of the Invention
This invention is directed to a modular humeral prosthesis which allows for total shoulder joint replacement and reconstruction of the human humerus.
2. Description of the Prior Art
Humeral prostheses are known in the art and are used for replacing the shoulder joint in humans. These prostheses generally include an elongated stem portion disposed within the humeral shaft, and a prosthetic humeral head which replaces the natural humeral head and is disposed within the shoulder socket. Currently, an accepted method of treatment for three and four-part and humeral head split fractures, in which the blood supply to the fragments is compromised, is to perform a hemiarthroplasty, replacing the humeral head with a prosthesis. Experience has shown that hemiarthroplasty affords favorable results as compared to non-operative treatments. This is because the blood supply is often compromised to the humeral head fracture parts and osteonecrosis often occurs.
U.S. Pat. No. 4,865,605 to Dines et al discloses an example of such a prior art modular shoulder prosthesis. The prosthesis includes an elongated stem portion having a planar support surface which supports a semi-spherical humerat head at one end. One of the support surface and the humeral head includes a stem, and the other includes a bore. The stem and bore are both tapered to allow the head to be disposed on the shaft with a friction fit. A similar structure is disclosed in U.S. Pat. No. 4,191,670 to Dale et al.
In both Dale et al and Dines et al, the head and stem portion are modular, that is, they are made available in different sizes but with a uniformly-sized tapered stem and bore. Accordingly, an elongated stem portion of appropriate size may be selected, and each of the different-sized heads may be removably disposed on the selected elongated stem portion while it is disposed in the humeral shaft. This structure allows the surgeon to accurately balance the tension of the soft tissue about the shoulder during implantation of the prosthesis by intraoperatively evaluating different humeral head sizes. If it is necessary to make revisions to the size of the head, the revision can be made without having to remove the elongated stem portion. Though effective in total shoulder joint replacement, that is, where the humeral shaft is healthy and the prosthesis is inserted for the purpose of replacing the joint between the healthy humeral shaft and the shoulder, the devices disclosed in Dines et al and Dale et al are not ideal for treatment of humeral shaft fractures.
In contrast to the above-discussed prostheses, humeral nails or rods are designed specifically for humeral shaft fractures. Humeral nails or rods are generally cylindrical, and are inserted into the intramedullary canal of the fractured humerus. Once inserted, the nail or rod may be secured in place, for example, by cross screws extending through both the nail and the bone, or by a screw cap secured at the upper end. In order to make use of nails, both the humeral head and a portion of the humeral shaft extending at least 2-3 cm from the surgical neck of the humerus must be intact. Thus, humeral nails are of no use in treating fractures of the humeral head, or of the humeral head region.
Further, the prior art is not ideal for treating fractures of the humeral head in which it is desired to repair the fractured head. In some cases, where the blood circulation to the fracture fragments is adequate, the fracture fragments may be viable, and it is desirable to attempt repair of the humeral head. However, in order to make use of the above-discussed shoulder prostheses of Dine and Dale, the humeral head must be removed and replaced by the prosthetic humeral head which is inserted in the glenoid or shoulder socket. Thus, there is no provision for attempting repair of the humeral head.
Accordingly, it has been desired to manufacture a modular prosthesis which not only is effective for total shoulder joint replacement, but also allows for repair of humeral shaft fractures. That is, the device should allow for both separate or simultaneous treatment of humeral shaft and humeral head fractures. Such a prosthesis should allow for retention of the natural humeral head when it is used for treating humeral shaft fractures in situations where the head is healthy. The prosthesis should offer the surgeon the flexibility to attempt to repair of the fractured humeral head, while allowing a hemi or total arthroplasty to be performed at a later time should the repair prove unsuccessful. Further, the prosthesis should be effective in treating mid-shaft humeral fractures, with the prosthesis constructed so as to span the fracture in patients having longer or tapered bones, with a good fit within the intramedullary cavity of the humeral shaft.